1. The present invention relates to methods of indirect-evaporation cooling of fluids and to heat exchange apparatus for affecting these methods.
2. The invention can be used for air conditioning, as well as cooling liquids and gases in different technological processes. It can be used to cool materials that can be conveyed along the heat transfer surfaces of the apparatus by methods other than fluidization.
The use of Evaporative methods to cool gases is well-known. The use of adjacent channels or heat transfer services to allow an evaporation in one channel to provide cooling for material in the second channel is also well-known, see. Niehart 2,174,060.
The methods and apparatus to cool air through evaporation have proved useful over many years. However they have certain drawbacks and limitations due to their designs.
There is known in the art a method of indirect-evaporation cooling of air, comprising cooling the flow of outside air over a heat exchange apparatus (USSR Patent No. 979796).
The outside air is pushed over a heat transfer surface, or moisture proof plates of the Dry Channel. The apparatus is comprised of a number of vertical moisture-proof plates which divides alternately Dry Channels and Wet Channels. At the outlet from the Dry Channel the flow of air is divided into two flows, namely, the cooled product flow and working flow to the evaporation or wet channel. The cooled flow goes to the consumer, and the evaporative flow is directed in counter flow of the Dry Channel, in the Wet Channel. The flows are controlled by the creation of aerodynamic resistance at the Dry Channel outlet. The heat transfer between the dry and Wet Channels causes heat to be drawn out of the outside air in the Dry Channel across the heat transfer surface and into the evaporation of the water in the Wet Channel. Cooling the air by the heat transfer surface occurs from the inlet of the Dry Channel to the exit. This allows air temperatures at the end of the Dry Channel to approach the dew point temperature of the air entering the Dry Channel.
The essential disadvantages of the described method and the apparatus for effecting same are: 1) the Product Fluid can not be cooled even in an ideal case lower than the temperature of the dew point of outside air; 2) the impossibility of cooling materials other than air or gas and; 3) difficult to realize cooling process for use in vehicles.
In addition to the above indirect-evaporation cooler there is a conceptual method and design apparatus for Evaporating and Cooling Water disclosed in Maisotsenko patent USSR Patent No. 690271 and USSR Patent No. 641260 where by single pass of air is used to cool water. In this method and apparatus the outside air flow is pushed down a Dry Channel with a heat transfer surface between the dry and wet channels and turned 180 degrees at the end of the channel and pushed up in counter flow across the water wetted heat transfer surface. Evaporation of water from the Wet Channel then draws heat across the heat transfer surface cooling the air in the Dry Channel and also cooling the water in the Wet Channel. Enough water is drawn over the Wet Channel to allow evaporation and collection of cooled water at the bottom of the channel which becomes the cooled product. Cooling the air in the Dry Channel allows for water temperatures at the bottom of the channel to approach the dew point temperature of the outside air.
The essential disadvantages of the described method and the apparatus for effecting same are: 1) the water being cooled can not be cooled even in an ideal case lower then the dew point temperature of outside air; 2) The ability to cool only water; 3) This process does not use an induced draft exhaust system and; 4) The description of the materials and accessories needed to design and make the cooler make for impractical application.; 5) Cooling potential of this evaporation process is limited; 6) The heat transfer rate in the channels, especially the Dry Channels is low.
Rotenberg 5,187,946, which is copied from Russian patent 2046257 Maisotsenko, there is disclosed a Wet-Dry Channel heat exchange system with an evaporative cooler. This does not address the issues of the limitation of ambient air, the limited efficiency of this design or the separate product channel being cooled by the wet channel.
The use of desiccants in evaporative coolers is common, see Belding 6,050,100, where the desiccant dehumidifies the air, both the air that goes to a dry side of an indirect evaporative cooler and the air that is separated and sent to the wet side to evaporate the water and cool the dry side air flow for later use. The desiccant is by way of a desiccant wheel. Additionally, the use of the desiccant and separately treating the two air streams in Belding yields a primary stream for the dry side that is more humid and cooler than the drier and warmer secondary stream that is used for the wet side.
Unlike the disclosed invention herein, Belding does not use the same flow for the dry and wet side flows. As a result, the cooling is not great and there is no separation of product so only air can be cooled. Finally, the method requires complex components and separate treatment of the flows with added mechanics and energy requirements.
Lowenstein in 5,351,497 and his paper on xe2x80x9cSeasonal Performance of a Liquid Desiccant Air Conditionerxe2x80x9d ASHRAE Symposia 1995 makes use of liquid desiccant on the dry side of an indirect evaporative cooler. Similar to Belding, the dry side air is separate and is the cooled product air.
Lowenstein uses the liquid desiccant to dehumidify the desired air flow for a living area, and the evaporative cooling is used to aid in absorbing the latent heat that is released by the dehumidification.
Lowenstein""s absorber, throughout makes use of liquid desiccant for dehumidifying air, does not make use of the unique feature of the within application. It does not give the advantages of lower temperature and controlled humidity.
Separate absorbers, using liquid desiccants were also discussed in Martinez and Khan, xe2x80x9cHeat and Mass Transfer Performance Analysis of a Compact, Hybrid Liquid Desiccant Absorberxe2x80x9d, 1996 IEEE. The discussion teaches a result contrary to the within disclosure that such an absorber could not be used alone to condition and cool air for living space.
The objectives of this invention is to make an improved method and apparatus for evaporation of a liquid to provide cooling for gases, liquids or other materials. The invention allows for cooling to a lower temperature than other methods. Its further objective is to make use of the cool product gas flow to cool other materials in an improved way without adding vapor or humidity to the product.
Further objectives of the invention is to make use of drying agents or desiccants to enhance the efficiency of the invention and its ability to cool. A further innovation is to make use of solid desiccants on a membrane or substrate to allow transpiration of vapor and fluid that is absorbed in the dry channel by the desiccants and then released in the wet channel by evaporation processes and thus cool the membrane and the dry channel.
The water vapor transpires through the solid desiccant and membrane.
Additional objects of the invention are to allow the desiccants to be concentrated and recycled to provide more efficiency to the cycle. The invention uses the recycling of the desiccant in combination with the use of the desiccant as part of the wet channel to accomplish both objects.
The main object of the invention is to provide an economical and environmentally safe method of cooling by indirect-evaporation and heat exchange apparatus, wherein the Product Fluid can be cooled to or lower than the dew point temperature of outside air. The object set forth is solved in different ways by using a core piece of heat and mass exchange apparatus in combination with the cooling process or processes that are desired. This core piece of apparatus can deliver cooling fluid by either producing cooled liquid or cooled gas.
The core of the apparatus passes Working Air along a Dry Channel with one side of a heat exchange membrane, then turns the flow 180 degrees and passes this same flow along a Wet Channel in counter flow with the same heat transfer membrane but on its opposite side. Evaporation cooling in the Wet Channel cools the Working Air in the Dry Channel. The Product to be cooled can be: 1. By the passing of the Product to be cooled through a third channel in heat transfer contact with the Wet Channel. 2. An excessive amount of Evaporative Liquid, (being drained off after cooling and passed through a Product Heat Exchanger like water, liquid desiccant, or liquid fuel 3, or other volatile liquid under the applicable pressures). A portion of the Working Air may be drawn out of the apparatus and used directly as a Product.
The unit can be built in a bank of channels. When the Product to be cooled is set in a channel along side the Wet Channel, it may also be in heat transfer contact with the Dry Channels due to the succession of units.
The fluid exiting the Wet Channel surface is considered the Exhaust. The difference in the total energy between the Working Air entering the Dry Channel and leaving the Exhaust is the Product cooling energy available. This is generally measured by the difference in enthalpy and flow. The Exhaust enthalpy is ideally limited by the Working Air temperature entering the Dry Channel at its corresponding saturation enthalpy.
The importance of pre-cooling the air before turning it to the Wet Channel and then obtaining lower temperatures can be understood by realizing that the Wet Channel Working Air starts at the lowest temperature attained in the Dry Channel, generally approaching the dew point temperature of the outside air. Before the Working Air enters the Dry Channel it""s temperature is generally at the outside air temperature. In all indirect-evaporation cooling apparatus for cooling outside air, other than described here, Working Air (used in evaporation) and product air temperatures start at the same point, the outside air temperature, forcing the temperature to approach the higher wet bulb temperature rather than the dew point temperature. Much lower temperatures can be realized with the use of desiccants to dry air in the Dry Channel, or by pre-cooling before going to evaporation as set out here, and lower the dew point temperature attainable.
The main differences between this apparatus and method and previous art are: 1. The means to create a workable method that will function in industry that is both efficient and economical to manufacture. 2. The wide use of fluid types in all channels. The Evaporative Liquid used in the Wet Channel for transpiration cooling can be any thing that will evaporate into the air under the ambient pressure and temperature. The Dry Channel and or the Product Channel of this method can also be a drying channel with the use of a desiccant on the heat exchange surface, or on a different surface within the Dry or Product channel, either liquid or solid to dry the air out while being cooled at the same time. This core design allows for many different types of fluids to be used and effective cooling at low cost. 3. The core method allows for a wider variety in design considerations for cooling different types of products. 4. The heat transfer surfaces on the walls can be varied from impermeable, to micro-sieve, or to perforated. Perforations or capillary channels allows for transpiration conductively from the Dry Channel to the Wet Channel. This has advantages in heat transfer and in efficiencies.
There are many variations that can be used with this core method of the invention that are described here after. The variations fit the wide variety of applications the core method can be used in.
It is always advantages to wet both surfaces of the Wet Channel, both the Dry Channel-Wet Channel heat transfer membrane or wall and the Wet Channel-Product heat transfer membrane or wall, (when a Product Channel is used,) to improve the heat transfer rate.
In high humidity climates it is sometimes advantages to heat the outside air entering or moving through the Dry Channel. At a higher heat, with no change in humidity, there is a greater latent heat capacity due to the ability to take on more moisture before saturation. It is approximately five times faster than the energy spent to gain higher temperatures.
Drying the air could be with desiccants such as Lithium chloride, bromide, calcium chloride, glycol, triethylene glycol etc. This allows cooling below the dew point temperature of outside air when combined with desiccants before or in the Dry Channel because it reduces the moisture content and thus increases the latent heat potential capacity.
In addition liquids, in the applicable pressure and temperature, with dew point temperatures less than that of water in air, such as gasoline, can be used in the Wet Channel. The fluid may be any suitable fluid that has a high vapor pressure at the ambient temperature and pressure so as to enhance the evaporation and thus take the heat of transformation from the remaining fluid.
The Working Air can be dried with a desiccant and then passed through the Dry and Wet Channels. This has the dramatic effect of reducing the temperature of the Working Air and therefore the minimum temperatures that can be obtained. The Product cooling available is the difference between the total energy, enthalpy and flow rate, of the hot Working Air in the Dry Channel and the total Exhaust energy leaving the Wet Channel.
The method can be effectively used to cool water for power plants and other typical cooling applications with return water temperatures closer to the dew point temperature of the outside air rather than the wet bulb temperature. In this case the Working Air is precooled in the Dry Channel and humidified in the Wet Channel. In many uses the temperature of the water to be cooled is warmer than the outside air. This added heat works to the coolers advantage as the temperature of the Working air will be increased which will also increase the available cooling energy. The use of desiccants to dry the air would lower the temperatures of the cooling water further as this is a greater capacity.
The liquid desiccant can be placed in the Dry Channel increasing the heat transfer rate from air to desiccant and desiccant to the heat transfer surface by five to ten times. This allows: 1. The temperature of the exhaust air to more closely approach the air temperature entering the apparatus. 2. The relative humidity of the exhaust to approach the saturation point, and, therefore, increases the energy available for cooling of the product.
Heat pipes can be used between the Wet and Dry Channels, and the Product Channel as well, if desired, creating the need for only one channel for each. This allows for easier configuration of a purely counter flow arrangement of the method.
In addition, using a desiccant in the Dry Channel provides continuous cooling of the desiccant and therefore increasing its absorption capacity and rate, drying the air faster and to a lower temperature.
Regeneration of the desiccant can take place within the core method with the use of a porous heat exchange panel between the Dry and Wet Channels. This panel would be designed to allow water absorbed by the desiccant to be drawn directly to the waterside through the panel by means of: 1. The lower pressure on the waterside. A pressure drop does need to be created between where the Dry Channel ends and the Wet Channel begins. The dry side may have forced draft fans and the exhaust may have an induced draft fan to create a larger pressure drop. 2. By the lower density of water on the waterside causing the water in the desiccant to want to move to the Wet Channel. 3. By the direction of the heat flow to the waterside. The porosity may be to water in liquid or vapor phase.
Depending on the Product to be cooled the Product Channel could have a desiccant used for drying and cooling the product as well.
The regeneration process does have a small loss in energy due to de-mixing of water moving from the desiccant to the Wet Channel, from liquid to liquid, but not going through a phase change. When the exhaust air absorbs this water a vapor change would take place creating a positive energy flow for cooling. The heat transfer rate will be larger due to the lack of a boundary layer in the channel separation wall and the direct connection of water to both sides of the membrane.
Solid desiccants can be used for the heat transfer surface in the regeneration process. Dry desiccants have the advantage of no heat loss from cooled desiccant flowing from the Dry Channel and carrying off some of the cooling energy. However the heat transfer rate from the Working Air to the desiccant is less with a dry desiccant. This desiccant regeneration and drying system could also be used in the Product Channel.
The core method can be efficiently used for air conditioning and cooling systems where liquid fueled engines are used such as in a vehicle. The Evaporative Liquid in the Wet Channel becomes fuel. The Dry Channel takes in outside Working Air for pre-cooling and passing through the Wet Channel. The Product Channel in heat transfer contact with the Wet Channel is cooled. In addition, it is possible to use a solid or a liquid desiccant in the Dry Channel, and liquid fuel and water to the working air in the Wet Channel simultaneously increasing the potential energy of cooling, due to the increased vapor pressure. In addition the fluid in the Dry Channel can be heated for the Dry Channel, before, during or at the end, with exhaust gases from the engine to provide additional vapor potential and thus, latent heat capacity of the working air and cooling product when water is being added to the Wet Channel with the liquid fuel. The desiccant can be re-concentrated with the heat source being the exhaust gas of the engine.
The addition of water in the Wet Channel will produce water vapor in the fuel-air mixture, which is directed to the engine, and it helps to improve the combustion process in the engine of a vehicle.
With vehicles that do not use enough fuel to cool a vehicle, or electric vehicles, core method with water/desiccant system can be used.
Creating a lower pressure in the Wet Channel will increase the vapor drive from water to the air. Increasing the pressure in the Dry Channel will increase the vapor drive to the desiccant. Pressurizing the Dry Channel and pulling a partial vacuum on the Wet Channel will require the insertion of a baffle between the channels to regulate the Working Air flow rate.
Recycling can be accomplished by use of liquid desiccants. Diluted liquid desiccants can be used in the Wet Channel with dry air to remove the water from the desiccants. This concentrated desiccant can then be used to dry the air either within the Dry Channel or outside the apparatus. To create a larger vapor drive difference between the Wet and Dry Channels, a pressure drop must be created between them. In addition the Dry Channel may need to have heat added before, during or after entering it and the Wet Channel may need to have water added to a surface both causing a greater vapor drive potential between the channels.
The method may be used to create cool concentrated desiccant for drying air and then using a more conventional cooling system in another process.
The core method can be efficiently used when working air is redirected from the Dry Channel into and through the Wet Channel, for example, through a plurality of spaced perforations or permeable pores formed in the heat exchange surface.
It can help to increase the coefficient of heat transfer between flows of working air in the Dry and Wet Channels. Also, it can help to transport absorbed water (when we use solid desiccant material) from the Dry to the Wet Channel.
The working heat exchange apparatus for effecting the above-described method will have: 1) A jacket with inlets and outlets for the Product Fluid and the Working Air or other fluids respectively. 2) The Product Channels for the Product Fluid. 3) The communicable Dry and Wet Channels for the Working Air with a heat exchange plate with or without perforations or pores. 4) The Product and Dry and Wet Channels are alternated and separated with plates. 5) A liquid distributor for channels with moisture on the walls such as a liquid desiccant or water. 6) Collecting trays for this liquid. 7) Valves for proper regulation of fluids in the channels. 8) Other components needed for specific function and operation of the apparatus, if pressure regulation is needed.
Counter flow is theoretically the most efficient design, however there are many designs that can be used to produce a more economically viable units using cross flow or some other combination of flow.
The plate or membrane, which is the heat exchange surface between the channels, can be made of wick, plastic, metal or solid desiccant materials or compositions of these materials.
While the description of this apparatus incorporates vertical channels for liquid wetting of airflow throughout the channel, there are various methods of moving liquids such as wicking, high air or vapor velocity, enough partial vacuum to lift the fluid, inclined slopes, etc. Depending on the application and design, the apparatus can be used with panels turned from horizontal to vertical.